There are various known processes for preparing benzoquinone which comprise oxidizing a phenol compound with oxygen in the presence of a metallic ion catalyst, such as Cu, Mn, or Co. They include, for example, those described in the specifications, of Japanese Patent Publication Nos. 17585/1978, 52889/1981, and 37255/1986, Japanese Patent Laid-Open Nos. 12334/1979 and 48726/1979, and U.S. Pat. No. 4,257,968, 4,360,469, and 4442036.
Among them, e.g., Japanese Patent Laid-Open No. 48726/1979 discloses a process which is relatively suitable for practical use, i.e., a process for preparing benzoquinone which comprises oxidizing phenol with oxygen in the presence of cupric chloride and lithium chloride.
However, this process is not very suited for practical use because of the difficulty in recovering the catalyst and the necessity of conducting the oxidation under pressure in an autoclave.
Further, there are disclosed various processes for preparing 2,3,5-trimethylbenzoquinone (hereinafter abbreviated to "TMQ") which comprise oxidizing 2,3,6-trimethylphenol (hereinafter abbreviated to "TMP"). These include, for example, those described in the specifications of Japanese Patent Publication No. 17585/1978 and Japanese Patent Laid-Open Nos. 93931/ 1975, 36641/1974, 92033/1974, 126636/1974, 14641/1975, and 72136/1980. However, these processes also have various drawbacks. For example, the catalysts used in these processes exhibit only a low catalytic activity in an aqueous solution. Further, because of the use of an organic solvent which is easily soluble in water, the catalyst must be dissolved in water in recovering the product, which makes it very difficult to recover the catalyst from the resulting aqueous solution.
Further, Japanese Patent Laid-Open Nos. 225137/ 1984, 81135/1985, 255745/1985, 255746/1985, and 17532/1986 each disclose a process in which the oxidation is conducted with a copper-halogeno complex (e.g., LiCuCl.sub.3, KCuCl.sub.3, or NH.sub.4 CHCl.sub.3) in the presence of an alkali metal halide. However, this process as well has drawbacks, e.g., that the preparation of a catalyst, i.e., a copper-halogeno complex crystal, is troublesome and that the use of a high-boiling higher aliphatic alcohol as the solvent in the reaction makes this process very disadvantageous with respect to the removal of the solvent at the time of isolating TMQ from an extract after the reaction, which renders this process commercially unsuitable.
The present inventors have made extensive and intensive studies with a view to developing a process which can eliminate the drawbacks of the above-detailed prior art and is more advantageous from the commercial viewpoint. Specifically, the present inventors have made studies for many years on a process which meets the three following requirements: 1 use of a catalyst comprising a combination of commercially available common reagents rather than the use of an expensive special catalyst; 2 use of a catalyst which exhibits a high catalytic activity, undergoes no deterioration of the catalytic activity and can be repeatedly used; and 3 use of a solvent system for reaction which enables not only the formation of a product in a high yield but also facilitates the isolation and purification of the product.